Exhaust aftertreatment systems are used to receive and treat exhaust gas generated by engines such as IC engines. Conventional exhaust gas aftertreatment systems include any of several different components structured to reduce the levels of harmful exhaust emissions present in exhaust gas. For example, certain exhaust aftertreatment systems for diesel-powered IC engines include a selective catalytic reduction (SCR) system which includes a catalyst formulated to convert NOx (NO and NO2 in some fraction) into harmless nitrogen gas (N2) and water vapor (H2O) in the presence of ammonia (NH3). A reductant is often inserted into exhaust conduits communicating the exhaust gas to the SCR system and/or other components of the aftertreatment system.
Natural gas as a fuel for heavy duty engines is receiving attention due to its potential to reduce pollutant and greenhouse gas emissions. Generally, natural gas engines comprise diesel engines converted to operate on natural gas, for example operating the diesel engine on natural gas using spark ignition (SI) stoichiometric parameters. For example, some natural gas engines may comprise diesel engines spanning a range from 6.5 L to 12 L in displacement converted to operate as natural gas engines. Such natural gas engines may be operated using stoichiometric combustion with cooled exhaust gas recirculation and three-way catalysis. However, simply converting diesel engines to operate on natural gas may cause the engine to experience high thermal stresses; relatively low efficiency due to low volumetric efficiency and compression ratio; unequal backpressure on engine cylinders, which may cause knock; and poor performance in terms of power and torque density, and transient response.